From 4199c42fe2b6fd9e106eccfc20035f65120d9143 Mon Sep 17 00:00:00 2001
From: Calvin Rose <calsrose@gmail.com>
Date: Wed, 4 Dec 2019 16:40:53 -0600
Subject: [PATCH] Add support for nested quasiquotation.

This brings Janet more in line with Scheme,
Common Lisp, and Clojure.
---
 src/core/specials.c | 27 +++++++++++++++++++--------
 test/suite7.janet   |  5 +++++
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/src/core/specials.c b/src/core/specials.c
index 1f82b099..83c686b1 100644
--- a/src/core/specials.c
+++ b/src/core/specials.c
@@ -55,7 +55,11 @@ static JanetSlot qq_slots(JanetFopts opts, JanetSlot *slots, int makeop) {
     return target;
 }
 
-static JanetSlot quasiquote(JanetFopts opts, Janet x) {
+static JanetSlot quasiquote(JanetFopts opts, Janet x, int depth, int level) {
+    if (depth == 0) {
+        janetc_cerror(opts.compiler, "quasiquote too deeply nested");
+        return janetc_cslot(janet_wrap_nil());
+    }
     JanetSlot *slots = NULL;
     switch (janet_type(x)) {
         default:
@@ -66,11 +70,18 @@ static JanetSlot quasiquote(JanetFopts opts, Janet x) {
             len = janet_tuple_length(tup);
             if (len > 1 && janet_checktype(tup[0], JANET_SYMBOL)) {
                 const uint8_t *head = janet_unwrap_symbol(tup[0]);
-                if (!janet_cstrcmp(head, "unquote"))
-                    return janetc_value(janetc_fopts_default(opts.compiler), tup[1]);
+                if (!janet_cstrcmp(head, "unquote")) {
+                    if (level == 0) {
+                        return janetc_value(janetc_fopts_default(opts.compiler), tup[1]);
+                    } else {
+                        level--;
+                    }
+                } else if (!janet_cstrcmp(head, "quasiquote")) {
+                    level++;
+                }
             }
             for (i = 0; i < len; i++)
-                janet_v_push(slots, quasiquote(opts, tup[i]));
+                janet_v_push(slots, quasiquote(opts, tup[i], depth - 1, level));
             return qq_slots(opts, slots, (janet_tuple_flag(tup) & JANET_TUPLE_FLAG_BRACKETCTOR)
                             ? JOP_MAKE_BRACKET_TUPLE
                             : JOP_MAKE_TUPLE);
@@ -79,7 +90,7 @@ static JanetSlot quasiquote(JanetFopts opts, Janet x) {
             int32_t i;
             JanetArray *array = janet_unwrap_array(x);
             for (i = 0; i < array->count; i++)
-                janet_v_push(slots, quasiquote(opts, array->data[i]));
+                janet_v_push(slots, quasiquote(opts, array->data[i], depth - 1, level));
             return qq_slots(opts, slots, JOP_MAKE_ARRAY);
         }
         case JANET_TABLE:
@@ -88,8 +99,8 @@ static JanetSlot quasiquote(JanetFopts opts, Janet x) {
             int32_t len, cap = 0;
             janet_dictionary_view(x, &kvs, &len, &cap);
             while ((kv = janet_dictionary_next(kvs, cap, kv))) {
-                JanetSlot key = quasiquote(opts, kv->key);
-                JanetSlot value =  quasiquote(opts, kv->value);
+                JanetSlot key = quasiquote(opts, kv->key, depth - 1, level);
+                JanetSlot value =  quasiquote(opts, kv->value, depth - 1, level);
                 key.flags &= ~JANET_SLOT_SPLICED;
                 value.flags &= ~JANET_SLOT_SPLICED;
                 janet_v_push(slots, key);
@@ -106,7 +117,7 @@ static JanetSlot janetc_quasiquote(JanetFopts opts, int32_t argn, const Janet *a
         janetc_cerror(opts.compiler, "expected 1 argument");
         return janetc_cslot(janet_wrap_nil());
     }
-    return quasiquote(opts, argv[0]);
+    return quasiquote(opts, argv[0], JANET_RECURSION_GUARD, 0);
 }
 
 static JanetSlot janetc_unquote(JanetFopts opts, int32_t argn, const Janet *argv) {
diff --git a/test/suite7.janet b/test/suite7.janet
index 2c2dc1da..2f92d2c0 100644
--- a/test/suite7.janet
+++ b/test/suite7.janet
@@ -265,4 +265,9 @@
   (assert (= (in buf 0) 0) "cryptorand doesn't overwrite buffer")
   (assert (= (length buf) 2) "cryptorand appends to buffer"))
 
+# Nested quasiquotation
+
+(def nested ~(a ~(b ,(+ 1 2) ,(foo ,(+ 1 3) d) e) f))
+(assert (deep= nested '(a ~(b ,(+ 1 2) ,(foo 4 d) e) f)) "nested quasiquote")
+
 (end-suite)